The present invention relates to improvements of an ignition coil, particularly for use in internal combustion engines for vehicles.
Conventionally, literatures such as JU-A-4849425, West German UM Registration No. 7924989, JP-A-59167006 and U.S. Pat. No. 4,546,753, for example, have presented a proposal wherein a permanent magnet is inserted in an air-gap portion of an iron core to increase energy stored in an electromagnetic coil such as an ignition coil. However, none of the literatures has disclosed an established technique relating to the structure of an ignition coil, as to what shape, dimension, etc. the iron core and permanent magnet in a magnetic circuit should have in order for the ignition coil to operate efficiently. In the past, even when a permanent magnet was used in an ignition coil put into practical use, a resulting ignition coil did not show any remarkable practical improvement in the performance and compactness as compared with an ignition coil using no permanent magnet. On the other hand, in recent years, a strong permanent magnet material containing such an element as samarium (Sm), neodymium (Nd), etc. has been developed and put into mass production, thus making it possible to expect expanded practical applications thereof. A permanent magnet made of such a material can have a strong magnetizing force capable of causing an iron core of an ignition coil to be saturated sufficiently when the permanent magnet is used to be inserted in a air-gap portion of the iron core of the ignition coil. Under the circumstances, permanent magnet materials having a property suitable for the application to ignition coils have become easily available.